Direct current (DC) motor drives in conjunction with Shape Memory Alloy (SMA) actuators are being explored for a wide range of automotive and similar applications ranging from power seats to power sunroofs to power shutters. These drives combine the best attributes of DC motors (low cost, high continuous power output, reversible motion) and SMA actuators (low mass, extremely small package size, high energy density) to allow a single DC motor to be multiplexed across multiple applications thereby yielding compact and low cost alternatives to the current practice of driving each application with a dedicated DC motor. SMA actuators serve to engage/disengage clutches that control the flow of torque and power from the DC motor to various loads. Smooth engagement requires the driving and driven members of the clutch to align properly and attain the same speed before engagement. Typically, friction cone extensions of the mating clutch elements or software based techniques are used to achieve this. Unfortunately, friction cone extensions do not perform well at small length scales, and software solutions introduce an undesirable time lag in clutch response.
To overcome the above drawbacks, magnetorheological fluids (“MRF”) have been proposed for use in a clutch. An MRF has a viscosity which can be controlled by applying a magnetic field. In the absence of a magnetic field, an MRF has a low viscosity. When a magnetic field is applied, the MRF viscosity increases substantially and can transmit torque and power through the viscous fluid.
Unfortunately, however, MRFs tend to exhibit long-term degradation when subjected to high shear stress in the viscous state.
There is thus a need for a clutch that either benefits from the controllable viscosity of the MRF while minimizing the stress on the MRF, or which is able to avoid the need for the MRF altogether. This goal is met as disclosed herein.